When utilizing solar arrays to obtain power from solar radiation, it is desirable to provide optimum conversion efficiency and to minimize deleterious thermal and optical effects. These objectives are particularly significant when solar arrays are incorporated in satellites or spacecraft. One approach to increasing efficiency is to lower operating temperatures of conventional solar arrays. This temperature reduction is achieved by decreasing the unusable solar energy that is absorbed, or by increasing the emissivity which is directly proportional to the energy that is reradiated into space. It has been shown that a power gain of 1% can be realized with a 0.02 decrease in solar absorption of an array, or by an 0.04 increase in front side emissivity of a two-sided array, or by an 0.02 increase in emissivity for a one-sided body mounted array.
Emissivity is enhanced by suppression of the coverglass reststrahlen reflectance and solar absorption is reduced by external reflection of the ultraviolet portion of the solar spectrum.
In prior art apparatus, solar cell coverglasses and thin film coatings have been provided to increase emissivity and to decrease solar absorption of the solar cell assembly. Such coverglasses are made from materials, such as fused silica or specialty glasses like Corning 0211 microsheet and Pilkington ceria doped glass. In conventional solar cell assemblies, thin film coatings are generally applied to the internal and external surfaces of the coverglass. The external coatings are usually formed as a single layer magnesium fluoride anti-reflection (AR) layer; whereas the internal coatings are ultraviolet reflecting coatings (UVR) which protect the adhesive, that is used for holding the coating to the substrate, from ultraviolet light exposure. Otherwise the ultraviolet rays would cause darkening of the adhesive thereby resulting in a reduction of energy transmission. Coverglasses produced with ceria doped glass do not normally require the internal UVR coatings to prevent darkening since they are opaque to ultraviolet radiation.